Benzophenone is widely used as a component of photoinitiator systems in UV-curable inks and coatings, especially in combination with aliphatic amines, such as dimethylaminoethanol or aromatic amines, such as dimethylaminobenzoates. However, the UV absorption of benzophenone is at a wavelength too short to give a good match with the emission of typical UV-bulbs, especially when doped UV bulbs are used, which emit at longer wavelengths. Therefore, phenylbenzophenone, which has an extended p-electron system, is often used to increase the absorption wavelength of photoinitiator systems, and to get a better match with UV-bulb emissions.
However, phenylbenzophenone (PBz) is a crystalline solid which exhibits limited solubility in acrylates, and which furthermore has a high melting point, and tends to crystallize from inks, coatings and photoinitiator-blends. This is especially problematic in low viscosity coatings and inks, such as UV-overprint varnishes, UV-flexographic inks and UV-digital inks, in which small molecules crystallize more easily, and product stability is a challenge.
Moreover, it is also a challenge to use phenylbenzophenone at a higher level to provide outstanding UV-cure, because there is then an increased danger of seeding and precipitation, especially in acrylates, which have lower solubility power. Acrylates, such as, for example, propoxylated pentaerythritol tetraacrylate and di-pentaerythritol hexaacrylate, are often key components used to formulate UV-curable flexographic and low migration inks.
EP 2508574 describes radiation curable compositions comprising an ethylencially unsaturated compound (e.g. (meth)acrylate), and inert hydroxyl-terminated polyesters. The inert polyesters may be modified to include moieties possessing photoinitiator activity. The examples describe inert polyesters modified with benzoic acid derivatives.
WO 2015/010729 discloses 4-phenylbenzoyl benzoic acid esters substituted with a residue of a hydroxyl compound having one to six hydroxyl groups. These compounds are useful as photoinitiators in radiation curable compositions.
4-phenylbenzoyl benzoic acid esters have been described in the literature as sun blockers in lotions or protective films (see, for e.g. WO 2010/075946 and DE 3831920). These are generally highly viscous, or solid compositions.
There remains a need to find photoinitiators that can be used in high levels in low viscosity inks and coatings, to promote faster cure speed and an increase in productivity.